The gut microbiota and immune response are key components to host pathogen defense, yet they are in a delicate balance that is mediated by the host genetic background. Gut microbiota stimulate host gut immune cells, while the host genetic background mediates microbial diversity, possibly through the host's immune response. Questions remain about whether the host genetic background contributes more to the gut microbiota than the environment, and to the strength of the immune response to microbes. To reflect the genetically diverse human population, we adapted the heterogenetic, biomedical model threespine stickleback fish (Gasterosteus aculeatus) for host-microbe studies. We hypothesize that host genetic background contributes to the diversity and membership of the gut microbiota in a gene by environment interaction (GxE), and to the magnitude of immune response to microbiota. Aim 1: Test the hypothesis that while the microbiota is shaped by a GxE interaction, the genetic background contributes more to the microbiota than the environment. Sub-aim 1: To examine the extent of GxE interactions, compare the diversity of gut microbiota of oceanic (Oc) and freshwater (FW) wild stickleback. We predict that there will be higher variability in microbiota within FW populations than within Oc populations because they exhibit greater inter-population genetic variation than Oc populations. Sub-aim 2: Examine the extent that the environment contributes to the microbiota by comparing gut and environmental microbiota. We predict the host will shape the gut microbiota, and that FW microbiota will reflect more diverse environmental microbes. Sub-aim 3: Examine the extent that host genetic background contributes to the microbiota by raising FW and Oc populations in a shared microbial environment. We predict that while microbiota will become more similar, inter-population variation will remain due to differences in host genetic background. Aim 2: Test hypothesis that immune response elicited by microbes is dependent on host genetic background. Sub-aim 1: Compare gut neutrophils in Oc and FW populations of stickleback raised in the presence and absence of microbes. Based on our previous findings, we predict that Oc fish will elicit stronger immune response to microbes than FW. Sub-aim 2: Identify gut microbes that elicit strong immune responses in populations of stickleback with different genetic backgrounds. We predict FW will not have as strong of a response to individual microbes as Oc, but that there will be higher variability between freshwater populations. As an early career scientist at a primarily undergraduate institution, I train and mentor students and postdocs to become active and engaged researchers. Since many pathways are evolutionarily conserved from fish to humans, I anticipate that results from this R15 grant will provide evidence that host genetic background influences microbiota membership via the immune response to microbes. These studies will lay the foundation for future studies in the how the host genetic background shapes the microbiota to protect against pathogens.